The invention relates to a satellite communication system comprising a primary station, a communication satellite and at least one secondary station.
The invention further relates to a primary station for use in a satellite communication system, to a secondary station for use in a satellite communication system and to a method of transmitting data signals from a primary station via a communication satellite to a secondary station.
Such satellite communication systems may for example be used for transmission of television programs or Internet data (i.e. so-called IP packets) to a large number of secondary stations. Communications conducted by such satellite communication systems are susceptible to atmospheric conditions between the primary station (e.g. a ground station or hub)/secondary station (e.g. a satellite receiver) and the satellite. In particular, rains are known to cause attenuation of satellite communications that are for example conducted in the Ka-frequency band. This attenuation is often referred to as rain fade attenuation or rain fade or rain fading and can severely impact the availability of satellite services.
A satellite communication system according to the preamble is known from U. S. Pat. No. 4,941,199. This United States Patent discloses two possible solutions to solve the problem of rain fading and to increase the availability of satellite services: By controlling (i.e. increasing) the transmit power (EIRP) of the satellite communication system. However, increasing the transmit power of the satellite is not always possible, because a satellite is normally power limited.
By applying a more robust forward error correction coding (FEC) scheme, resulting in a decrease of the effective data rate.
Dimensioning the system in such a way that full transponder capacity can be used even in case of heavy rain fading by applying a robust FEC scheme (having a lower effective data rate) is problematic, because this leads to a waste of bandwidth. For example, if the data rate of a carrier is decreased to increase the availability of 1% of the population in a rainy area, also the data rate for the other 99% will be decreased. This is clearly a waste of satellite bandwidth.
In addition, the received power density within the coverage zone of a spot beam of a satellite is in general not uniform. The satellite receivers (i.e. the secondary stations) situated at the center of the spot beam enjoy a higher received power density than the ones situated at the periphery of the beam. Therefore, the ones in the center can receive a higher data rate due to the higher receive power, then the ones at the periphery.
For these reasons it is desirable that the data rate can be modified dynamically (i.e. that the applied FEC scheme can be modified dynamically on a per packet basis (TDMA)). When the center population of a beam or receivers with clear sky are addressed a relatively high data rate can be used (i.e. a relatively less robust FEC scheme can be applied). When either the receivers at the periphery or the ones affected by rain fading are being addressed, the data rate can be lowered (i.e. a relatively more robust FEC scheme can be applied).
Currently available demodulators and decoders for use in satellite receivers, such as the TDA8043 Satellite Demodulator and Decoder integrated circuit of Philips Semiconductors, are not capable of handling a dynamically changing (i.e. on a per packet basis) FEC scheme in real time.
It is an object of the invention to provide a satellite communication system as described in the opening paragraph which can dynamically modify the applied FEC scheme and in which there is no need for specifically adapted demodulators and decoders. This object is achieved in the satellite communication system according to the invention, which satellite communication system comprises a primary station, a communication satellite and at least one secondary station, the primary station transmitting data signals via the communication satellite to the secondary station, the primary station comprising a modulator for modulating the data signals onto at least two differently FEC-encoded frequency carriers, the secondary station comprising a tuner for tuning to one of the frequency carriers in dependence on a tuning signal received from the primary station, the secondary station further comprising signal quality determining means for determining at least an estimate of a signal quality of the frequency carrier presently tuned to, the secondary station comprising retuning request transmit means for transmitting a retuning request signal to the primary station when the estimate of the signal quality becomes lower than a predetermined threshold value, the primary station comprising determining means for determining a frequency carrier with a higher signal quality upon reception of the retuning request signal and tuning signal transmit means for transmitting the tuning signal indicating this frequency carrier to the secondary station. The data signals are modulated (e.g. QPSK-modulated) onto a number of differently FEC-encoded carriers. A secondary station that is tuned to one of the carriers determines (an estimate of) the signal quality, e.g. the signal to noise ratio or the bit error rate, of the received data signals (carrier). If this signal quality is lower/worse than a certain threshold value, the secondary station will send a retuning request to the primary station, indicating that it would like to tune to a different carrier with a more robust FEC scheme and a higher availability. The primary station will in response send a tuning signal indicating the different carrier to the secondary station and the secondary station will tune to the different carrier. In this way, the FEC scheme employed can be modified dynamically.
In an embodiment of the satellite communication system according to the invention the tuning signal is transmitted from the primary station via the communication satellite to the secondary station. In this way, the same infrastructure is used to transmit the tuning signal as is used for transmitting the data signals. The secondary station may comprise a separate tuner for receiving the tuning signal and other control signals. Alternatively, the tuning signal may be transmitted from the primary station to the secondary station in any other conceivable way, e.g. via a telephone line.
In another embodiment of the satellite communication system according to the invention the retuning request signal is transmitted from the secondary station via the communication satellite to the primary station. In this way, the return channel of the satellite communication system is used to transmit the retuning request signal. Alternatively, the retuning request signal may be transmitted from the secondary station to the primary station in any other conceivable way, via a telephone line.